AN0539 frequency and resolution options for PWM outputs

For example, if the period is chosen to be 100 TOSC PWM1 period = [PR1 + 1] x 4TOSC PWM1 duty cycle = DC1 x TOSC where PR1 = period register for Timer1 concantenated 10-bit value TOSC =

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The PIC17C42 is equipped with two high frequency Pulse Width Modulation (PWM) outputs In a pulse width modulated signal the period of the signal is (usually) kept fixed, while the duty cycle is varied In this application note, we will discuss options in selecting PWM frequency and resolution

This application note assumes that the internal clock is used for the time-base, which is typically the preferred setup Also, throughout this application note, PWM1 output is used in examples, Timer1 is assumed to be the time-base

Definition of terms:

Period of a PWM output is the duration after which the PWM pattern will repeat itself

Frequency of a PWM output is = 1/Period

Resolution of a PWM output is the granularity with which the duty cycle can be modulated

In the case of the PIC17C42, when using PWM1 with Timer1 as the time-base:

At 16 MHz oscillator frequency, TOSC = 62.5 ns The user can control the frequency of the PWM output by altering the 'period' value of the time-base For example, if the period is chosen to be 100 TOSC

PWM1 period = [(PR1) + 1] x 4TOSC PWM1 duty cycle = (DC1) x TOSC where PR1 = period register for Timer1

concantenated (10-bit value)

TOSC = oscillator period

Authors: Stan D’Souza and Sumit Mitra

Microchip Technology Inc

(PR1 = 18h), then PWM frequency is:

1/(100 x 62.5) ns = 160 kHz

Note however that the duty cycle resolution is a little less than 7-bits

Useful and Common PWM Modes

While a variety of period values can be selected, the fol-lowing modes would be most commonly used:

10-bit Mode: In this mode PWM duty cycle has full 10-bit resolution (maximum offered by the PIC17C42) The period register PR1 is set at FFh PWM period = 1024TOSC = 64 µs PWM frequency is 15.625 kHz The user must write both PW1DCH and PW1DCL to update PWM output See Appendix A for an example of code that modulates 10-bit resolution PWM output (PWM10.LST)

8-bit Hi-Resolution Mode: In this mode, the user has only an 8-bit quantity to write to the duty-cycle register Period register is set at 3Fh (63 decimal), such that PWM period is 256Tosc To write the 8-bit duty-cycle value, first the 8th bit is right shifted two bits The upper six bits are written to PW1DCH and the lower two bits are written to PW1DCL as follows:

;8-bit duty-cycle value is in WREG CLRF TEMP ;

RRCF WREG ; RRCF TEMP ; RRCF WREG ; RRCF TEMP ;Shift right twice ANDLW b'00111111 ;Mask off two-high bits MOVPF WREG,PW1DCH ;Write duty-cycle values MOVFP TEMP,PW1DCL ;

Note that in 8-bit, hi-resolution mode, maximum PWM frequency is attained For example, at 16 MHz clock, PWM period = 256TOSC = 16 µs; PWM frequency = 62.5 kHz See appendix B for an example code that generates 8-bit low high resolution PWM output (PWM8HI.LST)

PWM

Duty-Cycle Period PWM frequency = 1/period

AN539

Frequency and Resolution Options for PWM Outputs

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DC9

DC7

DC2

DC1 DC0

PW1DCH

PW1DCL

DC1 DC0

10-bit

8-bit low resolution

8-bit high resolution

Note: DC registers suited for 16-bit computation, using 10 MSb only

8-bit Low Resolution Mode

In this mode, the user still has only an 8-bit quantity to

write to the duty cycle register However, the desired

frequency of the PWM output is less, due to the nature

of the application For example, if the PWM output is

being used to drive a motor through a power stage, the

power transistors, (or devices) due to their switching

time, will prefer PWM frequency not to exceed a certain

frequency In the previous section, we derived an 8-bit

resolution PWM output at 62.5 kHz

To attain a low-resolution PWM output, PW1DCL is

always kept at zero The 8-bit value is written to

PW1DCH The period (PR1) is set at FFh (i.e., 256 TCY

equals 1024TOSC [15.625 kHz]) See Appendix C for an

example code that produces 8-bit low resolution PWM

output (PWM8LO.LST)

Choosing Resolution and Frequency of PWM

Output

Actually, the resolution and the frequency of the PWM

output is selectable within certain limits The user will

need to first define the requirements based on the

application There may be an upper limit to the

frequency if the PWM is being used to drive motors

On the other hand, if the PWM is being filtered to

generate an analog signal, higher frequency may be desirable In any case, the lowest frequency achievable (using internal clock for the timer) is (FOSC/1024) At

16 MHz oscillator input, the lowest PWM frequency possible is 15.625 kHz At resolutions less than 10-bit, higher frequencies are possible (Figure 3) For exam-ple, if 7-bit resolution is chosen, then the PWM frequen-cies can be 15.625 kHz, 31.25 kHz, 62.5 kHz or

125 kHz The reader will note that it’s how the 7-bits are placed within the 10-bit possible duty cycle value Conversely, if a certain frequency is desired, such as

44 kHz, then referring to Figure 1, resolution can be 8.5-bit or 7.5-bit or 6.5-bit etc

Summary

The frequency and resolution of the PWM outputs of the PIC17C42 can be traded off against each other to best suit the application The oscillator frequency can also be varied to adjust PWM frequency, if necessary External clock should be used as timer time-base to generate very low frequency PWM output

250

125

62.5

31.25

15.625

PWM resolution

PWM frequency vs resolution for FOSC = 16 MHz

Timer time-base = internal clock

At a given oscillator frequency

(16 MHz in this figure), a family

of curves represents the PWM freq/resolution

combinations possible

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MPASM 01.40 Released PWM8HI.ASM 1-16-1997 13:59:20 PAGE 1

LOC OBJECT CODE LINE SOURCE TEXT

VALUE

00001 TITLE “PULSE WIDTH MODULATION 8 BIT HIGH RESOLUTION”

00002 PROCESSOR PIC17C42

00003

00004

00005 #include “p17c42.inc”

00001 LIST

00002 ; P17C42.INC Standard Header File, Version 1.03 Microchip Technology, Inc

00264 LIST

00006

00000021 00007 PWM_HI equ 0x21

00000020 00008 PWM_LO equ 0x20

00000022 00009 TEMP equ 0x22

00010 ;

00000001 00011 F equ 1

00012 ;

00013 ;The user would generate a 16 bit value which is saved in ram

00014 ;locations PWM_HI and PWM_LO byte In 8 bit hi-res mode, the program

00015 ;transfers the 8 bit values to the lo Duty Cycle (DC) registers, to

00016 ;generate the required 8 bit hi-res PWM

00028 LIST

00029 ;

00030 ;

00031 ;This is a short program to demonstrate how to generate PWM outputs with

00032 ;8 bit resolution Since a 10Mhz crystal was used in the test system

00033 ;The max period = 256x100nS = 25.6uS or 39KHz This program

00034 ;keeps the period constant and varies the duty cycle (which corresponds

00035 ;to the most significant 10 bits of the 16 bit value PWM_LO&PWM_HI)

00036 ;This program is interrupt driven, i.e the update to the Duty cycle

00037 ;is done in the TMR0 interrupt, which then enables the pwm interrupt

00038 ;The period update is done during the pwm interrupt The pwm output

00039 ;ramps up from 0% to 100% duty cycle and then repeats The full

00040 ;sweep takes approx 13.3 secs

00041 ;

00042 ;

00043 ;

00044 ; Program: PWM8HI.ASM

00045 ; Revision Date:

00046 ; 12-12-95 Compatibility with MPASMWIN 1.30

00047 ;

00048 ;*********************************************************************

00049 ;

0000 00050 ORG 0

0000 C063 00051 goto start

00052 ;

0010 00053 ORG 0x10 ;vector for TMR0 interrupt

0010 00054 TMR0_int

0010 C054 00055 goto service_TMR0 ;service TMR0

00056 ;

0020 00057 ORG 0x0020 ;vector for pwm interrupt

0020 00058 pwm_int

0020 C046 00059 goto service_pwm ;service pwm only

00060 ;

0030 00061 ORG 0x0030

00062 ;

Please check the Microchip BBS for the latest version of the source code Microchip’s Worldwide Web Address: www.microchip.com; Bulletin Board Support: MCHIPBBS using CompuServe® (CompuServe membership not required)

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00063 ;initialize internal hardware to generate the pwm output

0030 00064 init_pwm8hi

0030 B802 00065 movlb 2

Message[302]: Register in operand not in bank 0 Ensure that bank bits are correct

0031 2910 00066 clrf TMR1, F ;clear timer 1

00067 ;used to “drive” pwm1

0032 B062 00068 movlw 62 ;set period=39khz

0033 0114 00069 movwf PR1 ; /

0034 B803 00070 movlb 3

0035 2922 00071 clrf TEMP, F ;TEMP = mask for pw1dcl

0036 6A21 00072 movfp PWM_HI,WREG ;get duty cyl hi byte

0037 190A 00073 rrcf WREG, F ;rotate hi through carry

0038 1922 00074 rrcf TEMP, F ;rotate into lo byte

0039 190A 00075 rrcf WREG, F ;repeat for 2nd lsb

003A 1922 00076 rrcf TEMP, F ; /

003B B53F 00077 andlw B’00111111’ ;mask hi bits

003C 4012 00078 movpf W,PW1DCH ;save in high

Warning[202]: Argument out of range Least significant bits used

003D 7022 00079 movfp TEMP,PW1DCL ;save in low

003E 2916 00080 clrf TCON1, F ;tmr1 inc internally

00081 ;as 8 bit counter

003F B011 00082 movlw B’00010001’ ;start tmr1 and

0040 4017 00083 movpf W,TCON2 ;enable pwm1

0041 B801 00084 movlb 1

0042 2917 00085 clrf PIE, F ;clr all int enables

0043 2916 00086 clrf PIR, F ;clear all interrupts

0044 8307 00087 bsf INTSTA,PEIE ;except peripheral int

0045 0005 00088 retfie

00089 ;

00090 ;

00091 ;everytime a new value is written to the PWM_HI, PWM_LO regs, the

00092 ;tmr1 interrupt is enabled The DC value are written just before

00093 ;the “pwm interrupt” is enabled Here the new period register is

00094 ;updated In this example, period is kept constant at 62 Tcyl

0046 00095 service_pwm

00096 ;if the period changed, write new value here

0046 B802 00097 movlb 2 ;select bank 2

0047 B062 00098 movlw 62 ;period = 62 Tcyl

0048 0114 00099 movwf PR1 ; /

0049 B801 00100 movlb 1 ;disable tmr1 int

004A 8C17 00101 bcf PIE,TMR1IE ; /

004B 0005 00102 retfie

00103 ;

00104 PAGE

00105 ;This part of the program is basically used to simmulate a change

00106 ;which would be used to drive the pwm output

00107 ;

00108 ;the TMR0 is set up to interrupt every 52 mS

004C 00109 init_TMR0

004C B020 00110 movlw B’00100000’ ;set up TMR0 timer

004D 650A 00111 movfp WREG,T0STA ; /

004E 290B 00112 clrf TMR0L, F ;clear TMR0

004F 290C 00113 clrf TMR0H, F ; /

0050 B031 00114 movlw 31 ;init pwm at 50%

0051 0121 00115 movwf PWM_HI ;save in high

0052 8107 00116 bsf INTSTA,T0IE ;enable TMR0 int

0053 0002 00117 return

00118 ;

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00119 ;Every TMR0 interrupt, the PWM_HI&PWM_LO bytes are incremented by 1

00120 ;Only the 8 most significant bits are incremented

00121 ;

0054 00122 service_TMR0

0054 8D07 00123 bcf INTSTA,T0IF ;reset int flag

00124 ;do a pseudo inc of the 8 bit PWM_HI

0055 1521 00125 incf PWM_HI, F ;inc PWM_HI

00126 ;now load the values into the Duty Cycle registers

0056 B803 00127 movlb 3 ;bank 3

005A 1922 00131 rrcf TEMP, F ;rotate into lo byte

005B 190A 00132 rrcf WREG, F ;repeat for 2nd lsb

005C 1922 00133 rrcf TEMP, F ; /

005D B53F 00134 andlw B’00111111’ ;mask hi bits

005E 4A12 00135 movpf WREG,PW1DCH ;save in high

005F 7022 00136 movfp TEMP,PW1DCL ;save in low

0060 B801 00137 movlb 1

0061 8417 00138 bsf PIE,TMR1IE ;enable tmr1 int

0062 0005 00139 retfie

00140 ;

00141 PAGE

00142 ;

0063 00143 start

0063 8406 00144 bsf CPUSTA,GLINTD ;disable interrupts

0064 E04C 00145 call init_TMR0 ;initailize the TMR0 tmr

00146 ;for test purposes

0065 E030 00147 call init_pwm8hi ;initialize 8 bit pwm

0066 C066 00148 loop goto loop ;spin wheels

00149 ;

00150

00151 END

MEMORY USAGE MAP (‘X’ = Used, ‘-’ = Unused)

0000 : X - X - X - XXXXXXXXXXXXXXXX

0040 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXX -

-All other memory blocks unused

Program Memory Words Used: 58

Errors : 0

Warnings : 2 reported, 0 suppressed

Messages : 11 reported, 0 suppressed

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APPENDIX B: PWM8HI.LST

MPASM 01.40 Released PWM8HI.ASM 1-16-1997 13:59:20 PAGE 1

VALUE

00001 TITLE “PULSE WIDTH MODULATION 8 BIT HIGH RESOLUTION”

00003

00004

00001 LIST

00002 ;P17C42.INC Standard Header File, Version 1.03 Microchip Technology, Inc

00264 LIST

00006

00000021 00007 PWM_HI equ 0x21

00000020 00008 PWM_LO equ 0x20

00000022 00009 TEMP equ 0x22

00010 ;

00000001 00011 F equ 1

00012 ;

00014 ;locations PWM_HI and PWM_LO byte In 8 bit hi-res mode, the program

00015 ;transfers the 8 bit values to the lo Duty Cycle (DC) registers, to

00016 ;generate the required 8 bit hi-res PWM

00028 LIST

00029 ;

00030 ;

00032 ;8 bit resolution Since a 10Mhz crystal was used in the test system

00033 ;The max period = 256x100nS = 25.6uS or 39KHz This program

00040 ;sweep takes approx 13.3 secs

00041 ;

00042 ;

00043 ;

00044 ; Program: PWM8HI.ASM

00047 ;

00048 ;**********************************************************************

00049 ;

0000 00050 ORG 0

0000 C063 00051 goto start

00052 ;

0010 00054 TMR0_int

0010 C054 00055 goto service_TMR0 ;service TMR0

00056 ;

0020 00058 pwm_int

0020 C046 00059 goto service_pwm ;service pwm only

00060 ;

0030 00061 ORG 0x0030

00062 ;

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00063 ;initialize internal hardware to generate the pwm output

0030 00064 init_pwm8hi

0030 B802 00065 movlb 2

0032 B062 00068 movlw 62 ;set period=39khz

0033 0114 00069 movwf PR1 ; /

0034 B803 00070 movlb 3

0038 1922 00074 rrcf TEMP, F ;rotate into lo byte

0039 190A 00075 rrcf WREG, F ;repeat for 2nd lsb

003A 1922 00076 rrcf TEMP, F ; /

003B B53F 00077 andlw B’00111111’ ;mask hi bits

003C 4012 00078 movpf W,PW1DCH ;save in high

003D 7022 00079 movfp TEMP,PW1DCL ;save in low

003E 2916 00080 clrf TCON1, F ;tmr1 inc internally

003F B011 00082 movlw B’00010001’ ;start tmr1 and

0040 4017 00083 movpf W,TCON2 ;enable pwm1

0041 B801 00084 movlb 1

0042 2917 00085 clrf PIE, F ;clr all int enables

0043 2916 00086 clrf PIR, F ;clear all interrupts

0044 8307 00087 bsf INTSTA,PEIE ;except peripheral int

0045 0005 00088 retfie

00089 ;

00090 ;

00092 ;tmr1 interrupts is enabled The DC value are written just before

00094 ;updated In this example, period is kept constant at 62 Tcyl

0046 00095 service_pwm

0046 B802 00097 movlb 2 ;select bank 2

0047 B062 00098 movlw 62 ;period = 62 Tcyl

0048 0114 00099 movwf PR1 ; /

0049 B801 00100 movlb 1 ;disable tmr1 int

004A 8C17 00101 bcf PIE,TMR1IE ; /

004B 0005 00102 retfie

00103 ;

00104 PAGE

00107 ;

004C 00109 init_TMR0

004C B020 00110 movlw B’00100000’ ;set up TMR0 timer

004D 650A 00111 movfp WREG,T0STA ; /

004E 290B 00112 clrf TMR0L, F ;clear TMR0

004F 290C 00113 clrf TMR0H, F ; /

0050 B031 00114 movlw 31 ;init pwm at 50%

0051 0121 00115 movwf PWM_HI ;save in high

0052 8107 00116 bsf INTSTA,T0IE ;enable TMR0 int

0053 0002 00117 return

00118 ;

Trang 8

00121 ;

0054 00122 service_TMR0

0054 8D07 00123 bcf INTSTA,T0IF ;reset int flag

00124 ;do a pseudo inc of the 8 bit PWM_HI

0055 1521 00125 incf PWM_HI, F ;inc PWM_HI

0056 B803 00127 movlb 3 ;bank 3

005A 1922 00131 rrcf TEMP, F ;rotate into lo byte

005B 190A 00132 rrcf WREG, F ;repeat for 2nd lsb

005C 1922 00133 rrcf TEMP, F ; /

005D B53F 00134 andlw B’00111111’ ;mask hi bits

005E 4A12 00135 movpf WREG,PW1DCH ;save in high

005F 7022 00136 movfp TEMP,PW1DCL ;save in low

0060 B801 00137 movlb 1

0061 8417 00138 bsf PIE,TMR1IE ;enable tmr1 int

0062 0005 00139 retfie

00140 ;

00141 PAGE

00142 ;

0063 00143 start

0063 8406 00144 bsf CPUSTA,GLINTD ;disable interrupts

0064 E04C 00145 call init_TMR0 ;initailize the TMR0 tmr

00146 ;for test purposes

0065 E030 00147 call init_pwm8hi ;initialize 8 bit pwm

0066 C066 00148 loop goto loop ;spin wheels

00149 ;

00150

00151 END

PULSE WIDTH MODULATION 8 BIT HIGH RESOLUTION

MEMORY USAGE MAP (‘X’ = Used, ‘-’ = Unused)

0000 : X - X - X - XXXXXXXXXXXXXXXX

0040 : XXXXXXXXXXXXXXXX XXXXXXXXXXXXXXXX XXXXXXX -

-All other memory blocks unused

Program Memory Words Used: 58

Errors : 0

Warnings : 2 reported, 0 suppressed

Messages : 11 reported, 0 suppressed

Trang 9

APPENDIX C: PWM8LO.LST

MPASM 01.40 Released PWM8LO.ASM 1-16-1997 13:41:45 PAGE 1

VALUE

00001 TITLE “PULSE WIDTH MODULATION 8 BIT LOW RESOLUTION”

00003

00004

00001 LIST

00002 ; P17C42.INC Standard Header File, Version 1.03 Microchip Technology, Inc

00264 LIST

00006

00000021 00007 PWM_HI equ 0x21

00000020 00008 PWM_LO equ 0x20

00000022 00009 TEMP equ 0x22

00010 ;

00000001 00011 F EQU 1

00012 ;

00014 ;locations PWM_HI and PWM_LO byte In 8 bit lo-res mode, the program

00015 ;transfers the 8 hi-byte value directly to the PW1DCH register

00027 LIST

00028 ;

00029 ;

00031 ;8 bit low resolution Since a 5.068Mhz crystal was used in the test system

00032 ;The max period = 1024x100nS = 102.4 uS or 9.8 KHz This program

00039 ;sweep takes approx 52 secs

00040 ;

00041 ;

00042 ;

00043 ; Program: PWM8LO.ASM

00046 ;

00047 ;*******************************************************************

00048 ;

0000 00049 ORG 0

0000 C053 00050 goto start

00051 ;

0010 00053 TMR0_int

0010 C04C 00054 goto service_TMR0 ;service TMR0

00055 ;

0020 00057 pwm_int

0020 C03E 00058 goto service_pwm ;service pwm only

00059 ;

0030 00060 ORG 0x0030

00061 ;

00062 ;initialize internal hardware to generate the output

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00063 ;for 8 bit low resolution pwm

0030 00064 init_pwm8lo

0030 B802 00065 movlb 2

0032 2B14 00068 setf PR1, F ;set period=9.8 khz

0033 B803 00069 movlb 3

0034 7221 00070 movfp PWM_HI,PW1DCH ;load duty cyl hi byte

0035 2910 00071 clrf PW1DCL, F ;clear lo byte

0036 2916 00072 clrf TCON1, F ;TMR1 inc internally

0037 B011 00074 movlw B’00010001’ ;start TMR1 and

0038 4A17 00075 movpf WREG,TCON2 ;enable pwm1

0039 B801 00076 movlb 1

003A 2917 00077 clrf PIE, F ;clr all int enables

003B 2916 00078 clrf PIR, F ;clear all interrupts

003C 8307 00079 bsf INTSTA,PEIE ;except peripheral int

003D 0005 00080 retfie

00081 ;

00082 ;

00084 ;TMR1 interrupts is enabled The DC value are written just before

00086 ;updated In this example, period is kept constant at 0xff Tcyl

003E 00087 service_pwm

003E B802 00089 movlb 2 ;select bank 2

003F 2B14 00090 setf PR1, F ;period < 0xff

0040 B801 00091 movlb 1 ;disable TMR1 int

0041 8C17 00092 bcf PIE,TMR1IE ; /

0042 0005 00093 retfie

00094 ;

00095 PAGE

00098 ;

0043 00100 init_TMR0

0043 B020 00101 movlw B’00100000’ ;set up TMR0 timer

0044 6500 00102 movfp W,T0STA ; /

0045 290B 00103 clrf TMR0L, F ;clear TMR0

0046 290C 00104 clrf TMR0H, F ; /

0047 B080 00105 movlw 0x80 ;begin PWM at 50% dc

0048 0121 00106 movwf PWM_HI ; /

0049 2920 00107 clrf PWM_LO, F ; /

004A 8107 00108 bsf INTSTA,T0IE ;enable TMR0 int

004B 0002 00109 return

00110 ;

00113 ;

004C 00114 service_TMR0

004C 8D07 00115 bcf INTSTA,T0IF ;reset int flag

00116 ;do a inc of the 8 bit PWM_HI

004D 1521 00117 incf PWM_HI, F

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